This study examines the effects of long-term ingestion of high fat diets on serum prolactin and on induced and spontaneous mammary tumors in rats. The relationship between fatty acid composition and hormonal and tumorigenic effects will be defined. Evidence that ingestion of a high fat diet enhances rat mammary tumorigenesis will be assessed, employing DMBA-induced tumors in rats fed one of several dietary fats of widely varying fatty acid composition. Serum prolactin is measured by RIA, and DMBA-induced and spontaneous mammary neoplasms are identified. The possibility of predicting ultimate tumor response from serum prolactin profiles will be determined by comparison of periodic serum prolactin paired values (proestrus-estrus: metestrus-diestrus) with tumor development. The distribution of prolactin receptors in various mammary lesions will be determined. Dietary fat effects on the pathogenesis and incidence of spontaneous or induced benign or malignant mammary tumors will be differentiated. Effects of dietary fat on weight gain, food utilization efficiency and maturation are monitored. Tumorigenesis is increased in rats fed 20% corn oil or lard compared to rats fed 5% fat; serum prolactin is not significantly altered by fat in young rats; studies of older rats are in progress.